1. Field of the Invention
The present invention relates to a power supply apparatus provided with a power supply circuit for generating a DC output.
2. Description of the Related Art
A power supply apparatus such as an AC/DC converter, a DC/DC converter, a charger, etc. is popular in various fields. Generally, a power supply apparatus should be small in loss. Particularly, it is very important to reduce loss in a power supply apparatus used in a portable personal computer, a terminal unit in a portable communications device, etc.
FIG. 1 shows the configuration of the charger or the DC power supply provided in the conventional power supply apparatus. The charger and the DC power supply basically have the same configurations, and individually include an electric power converter 510 and an analog circuit unit 520. In this example, the DC power supply is a DC/DC converter.
The electric power converter 510 includes a switching element (MOSFET) controlled according to the instruction from the PWM control circuit 524; a rectifying diode; an inductor for storing/discharging energy; a resistor for detecting an inductor current or an output current; and an output capacitor for smoothing an output. While the switching element is in an ON state, the inductor current is ramped up with the electric current provided for the load, thereby storing residual charge in the output capacitor. On the other hand, while the switching element is in an OFF state, the inductor current is ramped down and the electric charge stored in the output capacitor is discharged as necessary with the electric current provided for the load.
The analog circuit unit 520 includes an amplifier 521 for amplifying an inductor current; an amplifier 522 for amplifying the difference between the output from the amplifier 521 and a reference voltage Vref1; an amplifier 523 for amplifying the difference between the output voltage and a reference voltage Vref2; a PWM control circuit 524 for generating a PWM signal for controlling the switching element based on the output from these amplifiers, etc.; and an oscillator 525 for providing a clock at a predetermined frequency for the PWM control circuit 524.
When the output voltage becomes lower than the reference voltage Vref2, the PWM control circuit 524 sets to a high value the duty (duty cycle) of the PWM signal to be provided for the switching element so that the inductor current can be increased and the output voltage can become higher. On the other hand, when the output voltage becomes higher than the reference voltage Vref2, the PWM control circuit 524 sets to a low value the duty of the PWM signal so that the inductor current can be reduced and the output voltage can become lower. Thus, the output voltage can be can be maintained at a constant level. When the PWM control circuit 524 detects an overcurrent based on the output from the amplifier 522, it reduces the duty of the PWM signal or forcibly turns off the switching element.
Thus, an analog circuit has been used to control the output from a charger provided in the conventional power supply apparatus, or each DC power supply.
As described above, an analog circuit has been used to control the output from the conventional power supply apparatus. Therefore, the characteristics or specification of a power supply circuit cannot be easily amended. If they can be amended, a number of circuits have to be added for amendments. Considering a smaller or a lower cost power supply apparatus, the conventional technology has been impractical and unrealistic. Described below are examples of the problems with the conventional power supply apparatus.
(1) Precision in Output Voltage
Recently, high precision in voltage is required for a load (for example, the CPU of a computer). Therefore, a configuration is designed such that, to remove an influence of a voltage drop in the wiring between the output terminal of a power supply apparatus and a load, a voltage can be measured at an input terminal of the load, and the power supply apparatus can adjust the output voltage using the measured voltage as a feedback signal. Such a configuration can be referred to as a remote sense. However, there is a high possibility that a power supply apparatus which adopts the remote sense can generate oscillation. Providing an oscillation prevention circuit incurs an increase in the number of parts, thereby undesirably failing in making a small and low-cost power supply apparatus.
On the other hand, if the precision in voltage to be applied to an input terminal of a load is improved without adopting the remote sense, it is also necessary, in consideration of the fluctuation of the temperature around the power supply apparatus, to provide a compensation circuit for removing the temperature-dependency when a voltage drops. The compensation circuit can be realized as a circuit for generating a high-precision reference voltage, but it is normally expensive.
(2) Voltage Waveform when an Electric Power is Supplied or Disconnected
Semiconductor elements such as a TTL, a CMOS, etc. are widely used in a circuit having a load. Among them, a TTL element requires comparatively moderate regulations relating to the rise and decay speeds of an applied voltage. Therefore, it can be easily realized using a C-R circuit, etc. for the conventional power supply apparatus including an analog circuit. On the other hand, as well-known by one of ordinal skill in the art, a CMOS element requires a higher dv/dt (rise or decay speed of an applied voltage) around a threshold voltage. However, if a rise speed is simply increased for the conventional power supply apparatus, an overshoot occurs as it is commonly known. Thus, with the conventional power supply apparatus, a semiconductor element including a load can have a problem with its operation when it is turned on or off.
(3) Switching Noise
The power supply apparatus shown in FIG. 1 is normally called a switching regulator. As well-known, a switching regulator generates noise from its switching frequency. This noise may cause a malfunction of a peripheral circuit. Therefore, the conventional power supply apparatus has been equipped with a effective frequency filter for removing the noise, or a shield for protecting peripheral circuits against noise. These measures against noise are not recommended for a small or low-cost power supply apparatus.